1. Field of the Invention
The present invention relates generally to an automatic focusing apparatus, and more particularly, to an improvement of an automatic focusing-apparatus for automatically matching the focus relative-to an object in response to a video signal obtained from an image sensor, in an image sensing apparatus such as a video camera having an automatic focusing mechanism.
2. Description of the Prior Art
Conventionally, in an automatic focusing apparatus used in an image sensing apparatus such as a video camera, an approach utilizing a video signal itself obtained from an image sensor for evaluating the state in which the focus is controlled has been developed. According to such an approach, several good characteristics can be obtained. For example, there exists substantially no parallax. In addition, even if the depth of field is small and an object is located in the distance, the focus can be exactly matched. Furthermore, according to this approach, a specific sensor for automatic focusing need not be separately provided, so that the apparatus is very small as a mechanism.
As one example of such a focus control method utilizing a video signal, a control method referred to as a so-called hill-climbing servo system has been conventionally known. The hill-climbing servo system is described in, for example, U. S. Pat. Nos. 4,638,364 and 4,614,975 and Japanese Patent Laying-Open Gazette-Nos. 58505/1983 and 103776/1985. Briefly stated, a high frequency component of a video signal obtained from an image sensor is detected every one field as a focus evaluating value, the detected focus evaluating value is always compared with a focus evaluating value detected one field before, and the position of a focusing lens continues to be slightly vibrated such that the focus evaluating value always takes the maximal value.
In the above described hill-climbing servo system, if only the slope of a focus evaluating value is detected, the focusing lens is not stopped in the defocused position by driving the focusing lens in the direction of always increasing the focus evaluating value even if the object is changed, so that very good follow-up characteristics can be achieved.
Additionally, an automatic focusing apparatus having improved performance is proposed by one of the inventors of the present invention, which is disclosed in Japanese Patent Application No. 62-49512 filed Mar. 4, 1987.
FIG. 1 is a schematic block diagram showing the proposed automatic focusing apparatus, and FIG. 2 is a block diagram showing the details of a focus evaluating value generating circuit shown in FIG. 1.
In FIG. 1, a video camera comprises a focusing ring 2 for moving a focusing lens 1, a focusing motor 3 for driving the focusing ring 2, and an image sensing-circuit 4 including an image sensor (not shown) such as CCD (Charge Coupled Device). The focusing-lens 1 may be moved by a piezoelectric element instead of a motor. In addition, the image sensor not shown) itself such as the CCD instead of the focusing lens may be moved by the piezoelectric element.
An image formed on a surface of the image sensor by the focusing lens 1 is converted into a video signal by the image sensing circuit 4 and inputted to a focus evaluating value, generating circuit 5. Referring to FIG. 2 showing the details of the focus evaluating value generating circuit 5, a luminance signal component in a video signal outputted from the image sensing circuit 4 is applied to a synchronizing separator circuit 5a and a gate circuit 5c. The synchronizing separator circuit 5a separates a vertical synchronizing signal VD and a horizontal synchronizing signal HD from the inputted luminance signal, to apply the same to a gate control circuit 5b. The gate control circuit 5b sets a rectangular sampling area in a central portion of a picture in response to the inputted vertical synchronizing signal VD and horizontal synchronizing signal. HD and a fixed output of an oscillator,(not shown). The gate control circuit 5b applies a signal for opening or closing a gate every field to the gate circuit 5c such that passage of the luminance signal is permitted only in the range of the sampling area. The gate circuit 5c may be provided anywhere in the former stage of an integration circuit 5f as described below.
Only the luminance signal corresponding to the range of the sampling area is applied to a high-pass-filter. 5d every field by the gate circuit 5c. A high frequency component of the video signal separated by the high-pass filter 5d is amplitude-detected by a detector 5e, the detected output being applied to the integration circuit 5f. The integration circuit 5f integrates every field the detected output applied thereto, the integrated output being applied to an A/D converter 5g. The A/D converter 5g converts the integrated value inputted thereto into a digital value, to apply the digital value as a focus evaluating value in the current field.
Description is now made on an operation to occur immediately after automatic focusing-control is started. Immediately after an automatic focusing operation is started, a focus evaluating value corresponding to the first one field outputted from the, focus evaluating value generating circuit 5 is first applied to a memory 6 holding the, maximum value and a memory 7 holding the initial value, to be held therein. Thereafter, a focusing motor control circuit 10 rotates the focusing motor 3 in a predetermined direction. Thereafter., a comparator 9 compares the initial focus evaluating value held in the initial value memory 7 with the current focus evaluating value outputted from the focus evaluating value generating circuit 5, to generate a comparison-signal. Accordingly, the focusing motor control circuit 10 initialize the rotational direction of the focusing motor 3 in response to the comparison signal.
More specifically, the focusing motor control circuit 10 rotates the focusing motor 3 in the above described predetermined direction until the comparator 9 generates a comparison output indicating "large" or,"small". If and when a comparison output indicating that the current focus evaluating value is larger than the initial focus evaluating vale held in the initial value memory 7 is outputted from the comparator 9, the focusing motor control circuit 10 maintains the above described predetermined rotational direction. On the other hand, when a comparison output indicating-that the current focus evaluating value is smaller than the initial focus evaluating value is obtained, the focusing motor control circuit 10 reverses the rotational direction of the focusing motor 3.
In the above described manner, initialization of the rotational direction of the focusing motor 3 is completed. Thereafter, the focusing motor control circuit 10 monitors an output of a comparator 8. In order to prevent a malfunction due to noise of the focus evaluating value, the comparator 9 may be adapted not to generate-the comparison output indicating "large" or "small" while the difference between the initial focus evaluating value and the current focus evaluating value does not exceed a predetermined threshold value.
On the other hand, the comparator 8 compares the maximum focus evaluating value so far held in the maximum value memory 6 with the current focus evaluating value outputted from the focus evaluating value generating circuit 5, to output two kinds of comparison signals (S1, S2), that is, signals in first and second mode-in which the current focus evaluating value is larger or is decreased to be below a predetermined threshold value, as compared with the focus evaluating value held in the maximum value memory 6, respectively. If and when the current focus evaluating value is larger than the content of the maximum value memory 6, the content of the maximum value memory 6 is updated in response to the-output S1 of the comparator 8, so that the maximum value of the focus evaluating value so far is always held in the maximum value memory 6.
A signal indicating the position of a focusing ring is generated from the focusing ring 2 corresponding to the position of the focusing ring 2 supporting the focusing lens 1 and applied to a memory 13 holding the position of a focusing ring. The focusing ring position memory 13 is updated in response to the output of the comparator 8 such that the focusing ring position signal in which the focus evaluating value becomes the maximum is always held
The focusing motor control circuit 10 monitors-the output of the comparator 8 while rotating the focusing motor 3 in the direction initialized in response to the output of the comparator 9 as described above. When the comparison output S2 in the second mode in which the current focus evaluating value is decreased to be below the above described threshold value, as compared with the maximum focus evaluating value is obtained from the comparator 8, the focusing motor control circuit 10 reverses the rotational direction of the focusing motor 3. It is to prevent a malfunction due to noise of the focus evaluating value that the focusing motor 3 is not reversed until the current focus evaluating value is decreased to be below a predetermined threshold value.
After the focusing motor, 3 is reversed a comparator 14 compares the content of the focusing ring-position memory 13 corresponding to the maximum value of the focus evaluating value with the current focusing ring position signal generated from the focusing ring 2. When both coincide with each other, that is, the focusing ring. 2 is returned to the position, where the focus evaluating value is the maximum, the focusing motor control circuit 10 stops rotation of the focusing-motor 3. At the same time the focusing motor control circuit 10 outputs a lens stop signal LS. In the above described manner, a series of automatic focusing operations are completed.
A memory 11 and a comparator 12 are circuits for resuming an automatic focusing operation by the focusing motor control circuit 10 if the focus evaluating value is changed by more than a predetermined threshold value while the focusing lens is stopped. More specifically, a focus evaluating value at the time point when the automatic focusing operation by the focusing motor control circuit 10 is terminated so that the lens stop signal LS is generated is held in the memory 11. The comparator 12 compares, the content of the,memory 11 with the current focus evaluating value outputted from the focus evaluating value generating circuit 5. If the difference therebetween exceeds the predetermined threshold value it is considered that an object changed, so that the signal indicating that an object changed is applied to the focusing motor control. circuit 10. As a result, the automatic focusing operation by the focusing-motor control circuit 10 is resumed, so that an automatic focusing operation following the change of the object can be achieved.
However, the above described automatic focusing apparatus suffers from the following two disadvantages.
The first disadvantage is that since the rotational speed of the focusing motor 3 can not be increased, it is difficult to achieve a high-speed automatic focusing operation.
FIG. 3 is a graph showing the relation between the position of the lens (the distance between lens and the object) in the automatic focusing apparatus shown in FIG. 1 and the focus evaluating value. Referring now to FIG. 3, description is made in detail on the above described first disadvantage. In FIG. 3, an axis of abscissa represents the position of the lens and an axis of ordinate represents the focus evaluating value.
In FIG. 3, it is assumed that the automatic focusing operation is started in the state in which the lens is in a position A considerably spaced apart from an in-focus position P on the side of the object. In this case, the focusing motor 3 rotates in the direction of increasing the distance between the lens and the, object from the position A where the focus evaluating value is small and the object is significantly defocused, so that the focus evaluating value rapidly rises. When the lens reaches, the vicinity of a position B of the lens, the focus evaluating value gently rises. In addition, the lens passes through the in-focus position P, to reach a position P of the lens where the focus evaluating value falls below the above described threshold value. Thereafter, the lens is returned from the position P of the lens to the in-focus position P, to be stopped.
It is required that such a sequence of automatic focusing operations are performed at high speed. However, if the time which each comparator requires for comparison is reduced, a malfunction is liable to occur. Thus, the automatic focusing operations can be achieved substantially by moving the lens 1 by the focusing motor 3 at high speed, i.e., rotating the focusing motor 3 at high speed. However, in the process of the above- described movement of the lens from the position B to the position P through the position P', the change of the focus evaluating value is very small, so that it is inevitable that overrun occurs due to the inertia of the motor itself when the motor is reversed in the position P' of the-lens or the motor is stopped in the position P of the lens. The higher the rotational speed of the focusing motor 3 is, the larger this overrun becomes. As a result, the time is rather required until the lens reaches the in in-focus position P. Thus, in the conventional automatic focusing-apparatus in which the rotational speed of the focusing motor 3 is always held constant as described above, the rotational speed of the focusing motor 3 can not be set too high, so that a high-speed automatic focusing operation can not be performed.
A second disadvantage is the change of the focus evaluating value caused by interlaced scanning- More specifically, in the above described conventional automatic focusing apparatus, the position of, the lens is controlled such that a focus evaluating value obtained from a level of a high frequency component in a video signal is always maximum. However, since the video signal obtained from an image sensor is ordinarily subject to interlaced scanning, the positions of an even field and an odd field constituting one picture are shifted within the picture by one scanning line. Consequently, even if the same object continues to be recorded, the focus evaluating value changes every one field, so that the position of the lens where the focus evaluating value is the maximum becomes unclear.